The present invention relates to (a) processes for making aqueous solutions that form a visible gel, (b) aqueous solutions capable of forming a visible gel, (c) visible gels, (d) processes for selectively reducing water permeability of a subterranean formation, and (e) natural resource recovery systems.
High water-oil ratios are a frequent production well problem. The most widely practiced production well gel technology for water control employs cation-crosslinked polyacrylamide gels. However, there are a number of problems that limit universal use of existing gel technologies. For example, gelation times with cation-crosslinked systems are often tricky to control. In addition, polymer or gel is often dislodged by and/or backproduced with reservoir fluids causing (a) problems with surface facilities and (b) limited longevity of the treatment response. Furthermore, the use of heavily crosslinked or highly concentrated gels that may limit back production often decrease oil production significantly by decreasing the permeability of the subterranean formation to oil.
In response to these problems, a low viscosity, water-based polyacrylamide system was developed that preferentially flows into those zones or portions of the subterranean formation having a higher water saturation. The system employs an organic crosslinking agent that creates a covalently bonded gel structure. At polymer concentrations below 4,000 ppm, visible gels--as opposed to microgels--are not obtained. Even at polymer concentrations as high as 5,000 ppm, it is very difficult, if not impossible, to form visible gels with this system.
Unfortunately, microgels, in comparison to visible gels, tend to reduce water permeability less and exhibit a shorter longevity.